1. Field of the Invention
This invention relates to a method of machining a round screw thread and, more particularly, to a round screw thread machining method for machining a round screw thread with a tool the radius of which is smaller than the arc radius of the threaded portion.
2. Description of the Related Art
There are cases in which a round screw thread whose threaded portion has the shape of a circular arc (see FIG. 4) is machined using an NC lathe. In such cases, a tool (a button tool) is designated, an NC program which specifies the tool path for moving the tool along the profile of the round screw thread is created and the round screw thread is cut using this NC program.
In creating such an NC program for machining a round screw thread, the first step is to create the following NC statement, which uses a G-code (G92) of a threading cycle, as a subprogram: EQU G92UuWwIiFf;
where "U" is a word address for commanding a tool relief quantity u/2 (incremental) along the X axis at the thread end point, "W" is a word address for commanding screw length w (incremental), "I" is a word address for commanding a taper amount i of the tapered screw, this indicating a straight screw when the taper amount is 0 (i=0), and F is a word address for designating lead. In accordance with these threading cycle NC data, in a case where the threading cycle starting position (cycle starting point) is a point A (FIG. 5), a tool path pattern is specified for cutting the round screw thread at the designated lead f by moving the tool from point to point in the manner A.fwdarw.B.fwdarw.C.fwdarw.D and then returning the tool to point A.
In a case where the radius r of a round screw thread portion RST (FIG. 5) is greater than the tool diameter of a button tool TL, the circular arc of the round screw thread must be machined by dividing the machining into a number of separate runs. That is, machining must be performed by finding offset partitioning points Pi' (i=1, 2 . . .) which offset, by an amount corresponding to the tool diameter, points on the circular arc of the round screw thread portion RST which divide the arc into a predetermined number of arcs, then finding threading cycle starting points Pi (i =1, 2 . . .) corresponding to these offset partitioning points Pi', and moving the tool to each of the cycle starting points Pi, followed by calling the subprogram for that threading cycle. In other words, with regard to the cycle starting point P1, an NC statement is created ir: such a manner that the round screw thread having the designated lead f is cut by moving the tool TL from point to point in the manner EQU P1.fwdarw.P1'.fwdarw.P1".fwdarw.P1'",
by positioning NC data for moving the tool to point P1 and a subprogram call instruction, after which the tool is returned to point P1. Similarly, with regard to the cycle starting point P2, an NC statement is created in such a manner that the tool TL is shifted from point P1 to point P2, after which it is moved from point to point in the manner EQU P2.fwdarw.P2'.fwdarw.P2".fwdarw.P2'"
by a command for positioning at point P2 and a subprogram call instruction. Thus, cycle starting points Pi are successively designated and a threading cycle subprogram call command is assigned whenever such a designation is made, thereby creating the NC program.
FIG. 6 is an example of an NC program for a case in which the radius of the round screw thread portion fabricated by such a method is larger than the tool diameter In FIG. 6, (a) signifies a subprogram "0200" for a threading cycle having a screw length of "350", a taper amount of "17", and a lead of "50"; (b) is NC data for setting coordinates, (c) signifies machining with a tool whose tool number is "202" (this is a tool selection instruction), and (d) signifies an NC statement for moving the tool to the threading cycle starting point indicated by "X.sub.-- Z.sub.-- ", and executing the threading cycle from this cycle starting point by the subprogram "0200" called by a subprogram call command "M98P200".
In accordance with the conventional round screw thread machining method, the circumference of the round screw thread is partitioned into a predetermined number of arcs, all of the cycle starting points for the threading cycles corresponding to respective ones of the partitioning points are calculated at the desk, the cycle starting points calculated are successively designated, and a threading cycle subprogram call command using the G92 command is assigned to each cycle starting point, thereby creating the NC program.
With the conventional method, however, the operation for calculating the cycle starting points requires a great deal of time and calculation errors are easily made. In addition, the arrangement is such that each threading cycle subprogram using the G92 command is created in advance, after which the subprogram is called upon designating the cycle starting point. As a result, the NC statements are large in quantity, as shown in (d) of FIG. 5, so that the operation for creating the NC statements requires a large amount of time.
Accordingly, an object of the present invention is to provide a round screw thread machining method in which cycle starting points need not be calculated at the desk in a case where the radius of the round screw thread portion is greater than the tool diameter, thus making it possible to simply create NC statements for the threading cycles of a round screw thread.